Wood fibers contain complex hydrocarbon molecules. When wood is burned, several things must occur. Some heat must originally be applied to break down those very large molecules into smaller parts, such that some individual carbon and hydrogen atoms become broken free. Next, an adequate supply of air must be available such that the oxygen from the air can chemically combine with these free atoms. The carbon atoms combine with oxygen atoms to eventually create carbon dioxide. Pairs of hydrogen atoms combine with oxygen to form water vapor. These are the desired end products of burning.
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For these fuels with more complex molecules, it is very important for there to be appropriate conditions for the intermediate-sized molecules created in the first stage, to be further broken down into individual carbon and hydrogen atoms, for the burning process to complete itself.
It is often described that "three T's" are necessary to ensure this happening. Temperature, Turbulence and Time. High enough temperature must be present to break down the complex molecules to release the individual carbon and hydrogen atoms. Turbulence is necessary to make sure that the oxygen in the air gets a chance to spread around well enough so each carbon atom and hydrogen atom gets a chance to be near some oxygen atoms. Enough time must pass (while these conditions apply) such that the various chunks of molecules have the chance to break entirely down to individual carbon and hydrogen atoms. Under those conditions, ANY fuel will experience perfect combustion, with 100% of the fuel molecules being consumed in the burning process and with all the final combustion products being water vapor and carbon dioxide.
Unfortunately, this often is not the case when burning wood. Large pieces of wood interfere with air turbulence. A very hot, roaring fire causes such a strong draft that some of the incompletely burned fuel molecules are carried up away from the fire and heat (Temperature) before all the reactions can be completed.
Even more problematical is that many products sold today are so-called "airtight" woodburners, that are designed to severely restrict the amount of air that is allowed near the fire. The manufacturers of such products do this to allow control the size of the fire, but a necessary side effect is that regularly some of those complex molecules could not be entirely combusted because not enough oxygen was available where it was needed in the fire.
All three of the T's must exist at the same place at the same time. The situations just described are where one of the T's either didn't occur at all or it occurred at the wrong time. Under this condition, incompletely burned fuel rises in the draft gases, up and away from the fire. Around 180 different chemicals have been detected leaving the fire in this way, and they have collectively been given the name "creosote".
While in the heated gases above a fire, creosote is gaseous, generally a dark grayish color. As the gases from a fire rise, they cool off. Once those gases cool to below around 350°F, the various creosote chemicals start to condense out, to change from being a gas to being a black gooey liquid or a black solid.
Keep in mind that, if the three T's actually applied, combustion would have been complete, and no creosote would have been in the gases above the fire, so no creosote would exist to condense out anywhere. Old, inefficient fireplaces nearly always had all three T's apply, so they burned with very high Combustion Efficiency. They were terribly inefficient because they had terrible Heat Transfer Efficiency. Any burning that has high combustion efficiency will have smoke leave the chimney that is either completely clear or pure white, depending on the outdoor humidity. (The white color is actually the water vapor from the fire condensing as it leaves a chimney top.)
Burning that has poor combustion efficiency has a substantial amount of creosote in the smoke. As such gases leave the top of a chimney, the creosote condenses and is seen as black or gray smoke.
When woodburning was done in non-airtight products, the amount of creosote produced was relatively limited and the gases moved quickly up and out the chimney system. The creosote then condensed out in the air above the chimney top and would then settle on the ground and snow outside. The design concept of airtight woodburners necessarily creates much more creosote in the smoke, and the limited air supply causes the gases to then move up the chimney much more slowly. This allows the creosote to condense out on the relatively cool walls of the chimney. These conditions are such that very rapid accumulation is possible. At a trade show in the early 1980s, I was told by a chimney manufacturer that they once did a test program with an airtight product with an eight-inch diameter chimney, where so much creosote was created by that product that the brand-new chimney entirely closed shut in less than three weeks of use!
These creosote accumulations inside a chimney are not at all obvious to the owner. Such accumulations can get rather thick on the walls of a chimney before it starts affecting the performance of the airtight woodburner by blocking the smoke leaving.
It turns out that creosote on the inside walls of a chimney can catch fire. If it does, creosote burns ferociously, at a temperature sometimes described at 5,000°F or 6,000°F. This tremendous heat production by the burning of the creosote means that is ANY of the creosote in a chimney ever started on fire, ALL of it soon will be burning. In addition, such very high temperatures cause phenomenal draft in the chimney so burning pieces of creosote seem to be shot out of the top of the chimney. Finally, and most dangerous, that tremendously high temperature inside the chimney soon gets the chimney extremely hot, which may crack or damage it or radiate heat to nearby wooden parts of the house. Many chimney fires have often then caused general fires that have burned down entire houses.
For these reasons, it is important for anyone who burns wood to regularly check their chimney for creosote accumulations. It is also important to have a plan to try to stop a chimney fire if one ever starts.
We have heard stories where people claim to have climbed up on the roof of their house to put a metal pan over the top of the chimney or to throw snow from the roof down into the burning chimney. After having seen some chimney fires, these seem impossible to believe. The fire coming out the top of a burning chimney is moving so fast that no snow would actually go down into the chimney and no person could get close enough to place anything on top and anything placed on top would immediately be blown up and away anyway. Any method to stop a chimney fire must be done below it, to restrict or stop the source of air for the fire.
In this regard, an airtight product might have an advantage. By entirely closing its air supply intake, no additional oxygen should be able to get to the creosote fire, so it should go out.
As a second line-of-defense, the concept of Fire-Out is to modify the very first piece of smoke pipe above the woodburner to have a stub of 3/4 inch black iron pipe attached to its side, which would be connected to a cylinder of carbon dioxide, such as a standard fire extinguisher or a small industrial cylinder. A valve could be included, which is triggered by a overheat sensor up in the chimney. In operation, if the chimney got too hot, the valve would open, filling the chimney with carbon dioxide gas to eliminate the oxygen necessary for combustion. The creosote chimney fire would therefore go out. A follow-up caution does apply, though. After causing the fire to go out, the walls of the chimney will still be very hot. Once the carbon dioxide is shut off, air and oxygen will then gradually enter the chimney. If the walls are still hot enough, a new chimney fire could start.
This whole situation can be avoided by proper use of any woodburner and by proper maintenance of all chimneys to avoid extensive creosote accumulations.
C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago